The present invention generally relates to the field of slotted coaxial antenna designs. More particularly, the present invention relates to the design of a slotted coaxial antenna that allows for simultaneous DTV and NTSC broadcast with equal or less windload than present VHF only antenna designs.
The majority of Ultra High Frequency (UHF) antennas used in National Television System Committee (NTSC) antenna systems are slotted coaxial designs. UHF slotted coaxial antennas gained widespread use in NTSC broadcasting because of their above-average performance characteristics, such as excellent omni-directional azimuth patterns and smooth null fill. Further, because UHF slotted coaxial antennas do not have any components that protrude from it, such as batwings or dipoles and is simply cylindrical in nature, the total antenna area susceptible to wind forces is less than VHF only antennas. Thus, the windloads on UHF slotted coaxial antennas are substantially less than the windloads on VHF only antennas.
While the foregoing performance characteristics are also desirable for digital television (DTV) transmission, the more stringent antenna output performance standards of DTV transmission cannot be met with current slotted coaxial antenna designs. At the present stage of antenna development, the antenna output response performance across multiple channels, which was given little consideration in NTSC systems, is now an important parameter for DTV transmission.
For example, when used as television broadcasting antennas, slotted coaxial antennas are generally optimized to transmit signals for a specified television channel having a six MHz bandwidth. For NTSC transmission, the power distribution across this six MHz bandwidth is concentrated at three basic carrier frequencies; namely, picture, color and aural. Therefore, the performance of the antenna is critical only at these three carrier frequencies.
However, for DTV transmission, the power is equally distributed across a 5.4 MHz bandwidth span within the 6 MHz bandwidth. Therefore, the antenna""s performance is critical across the operating band. This means that the antenna""s elevation pattern must remain stable (i.e., unchanged) at all frequencies within the channel, and not just at isolated frequencies.
In addition, there are over 400 Very High Frequency (VHF) television stations that have already been assigned UHF DTV channels. As a practical matter then, the onset of DTV has thus complicated the antenna selection decision for broadcasters who must now operate DTV antenna systems simultaneously with their existing NTSC antenna systems. It is desirable therefore to provide a coaxial antenna similar in nature to a slotted coaxial UHF antenna that allows for simultaneous DTV and NTSC broadcast with equal or less tower windloading than present VHF only antennas.
Further, the environment around the antenna may vary from one direction to another. For example, the ground level in one direction may be higher than the ground level in another direction at that site. Accordingly, it is also desirable to provide an antenna wherein the beam tilts of the UHF antennas can be independently adjusted in different directions.
The antenna of the present invention satisfies to a great extent the foregoing need for an improved slotted coaxial antenna design.
In one aspect of the invention a slotted coaxial antenna constituting a replacement antenna useful as a DTV and NTSC antenna system, is provided. The slotted coaxial antenna comprises an elongated cylindrical hollow mast. The mast acts as an outer conductor.
On the outside of the mast is arranged a plurality of substantially equidistant longitudinally extending spaced slots. In one embodiment, there are four VHF coupled slots and four UHF coupled slots arranged in alternating fashion. Each slot is formed in the mast for the purpose of radiating electromagnetic energy. Alternatively and optionally, each slot in the cylindrical mast may be of varying width and length for the purpose of varying the radiating field and ultimately the usable bandwidth of the antenna.
On the inside, the mast coaxially surrounds a longitudinally extending VHF inner conductor, which consists of a VHF slotted coaxial antenna. In one embodiment, the VHF inner conductor is surrounded equidistantly by four UHF slotted coaxial antennas, which each acts as a UHF outer conductor contained within the mast. Interposed between each UHF slotted coaxial antenna is a VHF coupler (also totaling four) such that the four UHF antennas and the four VHF couplers share a common aperture, namely, the VHF antenna. Each coupler (VHF or UHF) is located on the inside of the mast between the ends of each longitudinal slot.
A significant result of this slotted coaxial antenna design is an antenna output response performance that is suitable for VHF high band and UHF DTV broadcasts.
Another significant result is the achievement of DTV and NTSC signal coverage with equal or less tower windloading than current VHF only antennas.
In another aspect of the invention an antenna is provided that includes an elongated hollow mast having an upper portion and a lower portion, a VHF coupled slot in an outer periphery of the mast, a UHF coupled slot in the outer periphery of the mast, and a feed point, wherein the location of the feed point is between the upper portion and the lower portion.
In another aspect of the invention, an antenna is provided that includes a means for transmitting a VHF signal and a UHF signal simultaneously from the antenna, a means for dividing the antenna into an upper portion and to a lower portion, and a means for feeding the UHF signal to the upper portion and to the lower portion of the antenna.
In another aspect of the invention a method for transmitting signals from an antenna is provided that includes transmitting a VHF signal and a UHF signal simultaneously from the antenna, dividing the antenna into an upper portion and a lower portion utilizing a feed point, feeding the UHF signal to the upper portion and the lower portion of the antenna, utilizing the feed point, and varying a beam tilt of the antenna by moving the feed point between the upper portion and the lower portion.
As a practical matter, the common aperture UHF/VHF high band slotted coaxial antenna of the present invention also results in substantial economic savings, since the broadcast of NTSC and DTV signals can be transmitted from one common antenna. Moreover, it is adaptable to existing slotted coaxial antennas.
There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract included below, are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.